arraypad: allow padding with zero entries (i.e. no padding).

skimage/util/arraypad.py

@@ -1085,7 +1085,7 @@ def _validate_lengths(narray, number_elements):
     normshp = _normalize_shape(narray, number_elements)
     for i in normshp:
         chk = [1 if x is None else x for x in i]
-        chk = [1 if x > 0 else -1 for x in chk]
+        chk = [1 if x >= 0 else -1 for x in chk]
         if (chk[0] < 0) or (chk[1] < 0):
             fmt = "%s cannot contain negative values."
             raise ValueError(fmt % (number_elements,))

skimage/util/tests/test_arraypad.py

@@ -518,5 +518,12 @@ def test_pad_one_axis_three_ways():
                   **kwargs)
 
 
+def test_zero_pad_width():
+    arr = np.arange(30)
+    arr = np.reshape(arr, (6, 5))
+    for pad_width in (0, (0, 0), ((0, 0), (0, 0))):
+        assert np.all(arr == pad(arr, pad_width, mode='constant'))
+
+
 if __name__ == "__main__":
     run_module_suite()